The present invention relates to a diversity system, in particular, relates to a diversity system for an angle modulated digital signal transmission for reducing the effect of multipath fading. There is not often obtained a line-of-sight propagation path in a mobile communication system in a large city due to buildings and other obstacles, thus, the received signal is the sum of many signals reflected by buildings and other obstacles. Accordingly, severe fading occurs frequently with vehicle movement. The severe fading causes deterioration of the average received signal-to-noise ratio (SNR) and thus degradation of the error rate performance. If we try to assure sufficient error rate performance in the fading environment, extravagant transmitting power is necessary.
It has been well known that diversity techniques are useful to reduce the effect of the fading. There have been known many diversity systems, some of which are a space diversity, a frequency diversity, a route diversity, a time diversity, an angle diversity, and a sight diversity. The present invention relates in particular to a space diversity, which can be sometimes modified to an angle diversity and/or a sight diversity. A space diversity utilizes in principle at least two separated antennas either at the transmission side or at the reception side, and the received signals on separated antennas are combined. The field of the present invention is further limited to a digital transmission system utilizing angle modulation including both phase-shift-keying (PSK) and frequency-shift-keying (FSK), that is to say, the present invention can not be utilized for an analog signal transmission such as voice. In order to demodulate the angle modulated signal, the present invention utilizes a differential coherent detection system. Accordingly, the field of the present invention is summarized as
(1) A space diversity,
(2) Angle modulation for a digital signal, and
(3) A differential coherent detection system.
Concerning a space diversity, there have been known at least three combining methods, which are (1) selection combining, (2) equal gain combining, and (3) maximal ratio combining.
In a selection combining, received signal envelope on each antenna is compared with one another, and the antenna which provides the highest envelope level is selected through a switch.
In an equal gain combining, received signals are simply combined after the received signal phases are controlled to be in phase.
In a maximal ratio combining, the received signal on all antennas are combined after the received signals are co-phased and weighted so that the SNR of the combined signal becomes maximum. The maximal ratio combining is the best one of the prior three space diversity techniques. FIG. 1 shows the block diagram of the prior maximal ratio combining. In FIG. 1, the receiver has two separated antennas 50a and 50b. The phase difference of the signals received on those antennas is detected by the phase detector 52, which in turn controls the phase shifter 53 so that the phase difference between two signals becomes zero. The pair of in-phase signals (A) and (B) are applied to the combiner 56 through the respective weight circuits 54a and 54b, which are controlled by the envelope detectors 55a and 55b, respectively, so that the SNR of the combined signal becomes maximum. Thus, the outputs of the weight circuits 54a and 54b are pA and qB, where 0&lt;p.ltoreq.1, and 0&lt;q.ltoreq.1. Those weighted signals pA and qB are combined by the combiner 56, the output of which is therefore pA+qB. The output of the combiner 56 is applied to the detector 58 through the amplifier 57, and the detector 58 provides the demondulated output signal as a base-band signal.
However, the prior space diversity systems mentioned above have the disadvantages that the structure of the apparatus is complicated. That is to say, a circuit for envelope detecting of all received signals is necessary in the selection combining and the maximal ratio combining, a circuit for detecting the phase difference is necessary in the equal gain combining and the maximal rato combining, a phase control circuit for providing the co-phased signals is necessary in the equal gain combining and the maximal ratio combining. Further, the selection combining necessitates means for detecting the envelope levels in all the branches and selecting the branch having the highest envelope level. Further, the maximal ratio combining requires means for weighting according to the each received signal envelope as mentioned in accordance with FIG. 1.
The complicated structure of the apparatus causes the operational unstability of the receiver and also increases in the price of the apparatus.